Method of stabilizing silica sols



Sept- 19, 1967 M. MlNDlcK ETAl.

METHOD OF STABILZING SILCA SOLS 2 Sheets-Sheet l Filed April 1'7, 1961 oo. om om o ow om o om om o o on; l) 7 wozunozou 1 u w m: 3.5m q rl l Qn( 0.7 J/Ib/b 2 0 om l een M. M m 0 DI l P n o w m U. ON- l N l om] S O E |4- m. w .W u Q28 m. s n S O 0 l l om. I 1 1 o w n B l 3 H AJ ooNn v l owl o E 2L I@ O INVENToRs: MORRl s MINDICK OOON sonww aouvlonanoo 'Y T T A LEWIS E. REVEN BYMQWM'M Sept. 19, 1967 M. MINDICK ETAL.

METHOD OF STABILIZING SILICA SOLS 2 Sheets-Sheet 2 Filed April 17, 1961 nite Park, ill., assignors to Nalco Chemical Company, Chicago, lll., a corporation of Delaware Filed Apr. 17, 1961, Ser. No. 103,425 11 Claims. (Cl. 252-313) The present invention relates to a process for stabilizing aqueous silica sols. More particularly, the invention is directed to a method of treating aqueous silica sols to produce products that are stable under acid conditions.

A number of methods are known by which aqueous sili-ca sols can be formed. In most of these methods the initial step is to remove all but a small portion of the alkali metal ions which are present in water glass (sodium silicate). The product is a colloidal system of polymerized silicic acid. One of the early methods of forming aqueous silica sols consisted of neutralizing a sodium silicate solution with a mineral acid. In this process it is necessary to remove substantially all of the salts formed in the neutralization reaction. Although the salts can be removed by dialysis or electrodialysis, these procedures are not suitable for use in large scale plant operations.

The most successful process from a commercial standpoint for preparing silica sols is described in the Bird Patent 2,244,325. In this process an alkali metal silicate solution is passed through an ion exchange material which removes most of the sodium ions and thereby forms silicic acid. A typical silicic acid solution produ-ced by the Bird method contains about 3% SiO2, 0.006% NazO, and about 0.003% sul-fate. It has been found that the acidic sols produced by the Bird process, as well as by other methods, will gel in a few hours unless the sols are stabilized by the addition of an alkali.

One substantial disadvantage of many prior art methods of preparing silica sols is that the sols are only produced as relatively dilute solutions. In the Bird patent, for example, it is suggested that sols may be concentrated by evaporation only to a silica content o-f about A sol containing only 15% silica has many disadvantages. Such a product, for example, is uneconomical to ship and store because of its high water content.

More recently, a method of concentrating sols to a silica content of about 35% was `disclosed by Bechtold et al. in U.S. Patent 2,574,902. In this process a heel is formed by heating alkali stabilized sols of the Bird type to a temperature above about 60 C. The heel consists of large particles or nuclei of polymerized silicic acid. When additional quantities of the dilute sol are slowly brought into contact with the heel, the added silica polymerizes on the nuclei forming much larger particles. By this means the diameter of a typical Bird sol particle is increased from about 1 to 5 millimicrons up to from 15 to 130 millimicrons.

It is believed that colloidal silicic acid contains units of silica having a plurality of functional OH groups. These functional groups are extremely reactive and tend to form linkages with each other Which in time produce a three-dimensional cross-linked polymer and -gel formation. If an amount of alkali metal ion is added to the sol at the time the sol is produced in the ratio of SiO2 to NaZO of about 10:1 through 100:1, however, the ions will be absorbed on the surfaces of the particles causing the particles to become negatively charged. The particles then repel each other and agglomeration is prevented. Under ordinary conditions such colloidal silicic acid sols are stable for an indefinite period of time against gelation.

States Patent O In U.S. Patent 2,577,484, the patentee, Joseph M. Rule, suggests that aqueous silica sols that are prepared according to the method of the Bird Patent 2,244,325 should also be treated with an anion exchange resin to give a sol that is substantially free of both cations and anions. Following the treatment of the dilute sol with the exchange resins, Rule forms a concentrated silica sol by the method described by Bechtold and Snyder in Patent 2,574,902. In making the heel sol, a suicient amount of a strong base is added to adjust the pH of the sol to between 6 and 9. A build-up of the sol is effected by adding fresh, unheated salt-free active silica to the heel sol while maintaining the pH in the range from 7.5 to 10.7 and by heating the combined sols above 60 C. The products produced by the Rule method have an SiO2 to M20 ratio in the range of rfrom :1 to 500:1. These materials are stable against gelation so long as the pH of the sol remains between 7.5 and 10.7. The sols, however, are not stable under acid conditions.

A unique method of concentrating sols is described in Reuter et al. U.S. 2,929,790. In this process the SiOzzNaZO ratio is controlled so that only a sufficient number of sodium ions are present to stabilize the sol. While this method is effective in producing a sol that is stable under alkaline conditions, it is not suitable where the product is to be used under acid conditions.

As has been pointed out, aqueous silica sols preferably are stabilized by adding to the sol an inorganic base such as sodium, lithium, potassium, rubidium, caesium, or ammonium hydroxide. A suicient amount of the base is usually added to raise the pH of the product to from 7.5 to about l1. As is indicated in the Bechtold and Snyder patent, the sols would not be stable Without the addition of a strong base. If the pH is then returned to the acid side, the product becomes extremely unstable. Ordinarily, such a material will have a useful shelf life of only 5 to 6 hours. Understandably, a -product of this type is not suitable for use under acid conditions.

It is widely accepted in the art that the presence of certain ions accelerate the gelling of aqueous silico sols. Methods of stabilizing sols have been proposed in which the sols are passed through both anion exchange resins and cation exchange resins.

Although the treatment of sols with ion exchange resins has proved to be helpful in preventing gelation, such methods have not been completely satisfactory. In the method described by Rule in U.S. Patent 2,577,484, for example, it is necessary to maintain the pH of the sol between about 7.5 and 10.7 by the addition of a strong base. If the sols that have been treated with ion exchange resins are not stabilized by the addition of the base, it has been found that the pH of these sols tends to rise after the exchange treatment. The increase in pH causes the sols to become relatively unstable after a relatively short storage period.

The principal object of the present invention is to provide an improved process for producing sols which are stable under acid conditions.

Another object of the invention is to provide an improved method of stabilizing sols by the use of ion exchange systems.

Other objects will become apparent to those skilled in the art from the following detailed description of the invention.

In general, this invention comprises the discovery of a -method for improving the stability of acidic aqueous silica sols. This method includes the steps of contacting the sol with an ion exchange system containing a hydrogen form strong acid cation exchange resin, allowing the thus treated sol to age -under certain specific conditions, and subsequent-ly again treating the aged sol with a hydrogen form strong acid cation resin. It is also essential that the sols be treated at least once with an anion exchange resin. The treatment with the anion exchange resin can take place either before the aging step, after the aging step, or both before and after the aging step. In a preferred embodiment of the invention, the pH of the sol is adjusted after the initial treatment with the ion exchange resin system.

The ion exchange resin system used in the first step of the process may be either:

(A) A hydrogen form strong acid cation exchange resin.

(B) A hydrogen form strong acid cation exchange resin in combination with a hydroxide form strongly basic anion exchange resin, or,

(C) A hydrogen form, strong acid cation exchange resin in combination with a free base form weak base anion exchange resin.

In the rst step, silica sols having an SiO2 concentration ranging from between 3% to as much as 50%, are ycontacted with any one of the ion exchange resin systems describedy above. The exchange operation may be conducted either as a batch operation using a resiny slurry, or in a preferred embodiment, as a column operation.

The second step of the process comprises the aging of the' sols treated by the ion exchange resin system. They treated sols `should age at least 1 hour at a temperature not greater than the boiling point thereof with a preferred aging step being conducted at a temperature range between 40 F. to 120 F. for at Ileast 4 hours and most preferably between 16 to 24 hours. f f

While the aging process is ymost preferably conducted for the times and temperatures specified, it will be understood that the invention contemplates aging periods of up to one week or more at temperatures as low as 34 F. Also, where a rapid aging process is desirable, it is also possible to heat the sols at elevated temperatures, eg., 120 F. to 180 F., thereby allowing the aging process to ybe completed in shortened periods of time `ranging from 1 to about 4 hours.

yIn the third step of `the process, the aged sols are again contacted with a -hydrogen form strong acid cation exchange resin. As in the first deionization step, the resin system may be either A, B, or C Alisted above, with -the proviso that the sols must be passed at least once through an anion exchange resin. The second deionization step may be accomplished using either a slurry or column technique. Ilf the sols are to be treated with both an anion and cation resin after the aging step, they may be contacted in any sequence, although the preferred sequence is to treat the sols initially with a cation exchange resin and then with the anion exchange resin. A further deionization technique involves the use of a mixed bed system which contains an intimate admixture of hydrogen form strong acid cation exchange resin and an anion exchange resin.

It is preferred that the anion exchange resin be a strong base resin which is capable of salt-splitting reactions and which also will remove weak acids` such as carbonio and Ilower molecular weight silicic acids from the sols. It is also within the scope of the invention, however, to use a Weak base anion exchange resin.

The anion and cation exchange resins can be used in series or in mixed bed. The strong acid cation exchange resins remove cations (predominantly Na). Strong base anion exchange resins remove all anion impurities including weak anions.

There are several strong a-cid resins that are commercially available which can be used successfully in the subject process. One such product is sold under the trademarks Nalcite HCR and Dowex 50. This product is described in U.S. Patent 2,366,007. The resin is made by the nuclear sulfonation of styrene-divinyl benzene beads. Similarly, there are many strong base resins which can be used in the process. Examples of the materials include the commercially available Nalcite SBR, Dowex 1, Dowex 2, Dowex 11, and Dowex 21K, Nalcite SBR and Dowex 1 and 2 are described in U.S. Patent 2,591,573. Each of the above exchange resins has a rquaternary ammonium type of exchange group. The quaternary ammonium anion exchange resins are highiy ionized and can be used over the entire pH range. They are capable of saltsplitting reactions which convert a neutral salt to its corresponding base. A typical weak base anion exchange resin that can be used in the process is Amberlite IR- 4B which is modified phenolformaldehyde polyamine condensate containing 14% nitrogen and 39.9% water in the basic form of the resin and having an exchange capacity of 10 milli-equivalents per gram of resin. The properties of the resin are described by Kunin and Meyers in the Journal of the American Chemical Society, volume 69, p. 2874, for 1947. Other anion exchangers are de'- scribed in U.S. Patents 2,422,054 and 2,438,230.

The sols produced by treatment with the ion exchange resin system in the first process step are acidic to varying degrees depending upon the particular resin system employed. It has been observed that both the pH and viscosity of these sols will tend to increase in storage.

It is desirable that the aging process be conducted usingwsols which do not iiuctuate in pH and whichr do not increase in viscosity. The aging process is most benecially conducted upon sols having a pH within the range of lfrom 1.5 to not more than 3.5 with a preferred pH ranging between 2.5 and 3.

When the hydrogen form strong acid cation exchange resin is used as the ion exchange resin system, ythe treated sol will have a pI-I of 2 or less. This low pH sol is admirably suited for the subsequent process steps of the invention. When the ion exchange resin system comprises a combination of a hydrogen form strong acid cation exchangeresin in conjunction with either a hydroxide form strong or free base form Weak base anion exchange resin, the pH of the resultant treated sol will range between 3 to about 5, depending upon the particular resin system employed and the degree of regeneration of the cation exchange resin.

Specifically, when an incompletely regenerated hydrogen form strong :acid cation exchange resin is used in cornbination with a hydroxide form .strong base anion exchange resin to treat the silica sols, the resultant pH will be approximately 5. Upon standing at room temperature, e.g., 72i5 F., for one week, the pH of the treated sol will gradually approach 6. With increasing pH there Will also be noted an increase in viscosity.

When a completely regenerated hydrogen form strong acid cation exchange resin is used in con-junction with a `hydroxide form strong base anion exchange resin, the sols treated with such a system will have a finished pH of about 3, which, upon storage for one week at room temperature, will tend to increase to about 5.

The completely regenerated strong acid cation exchange resin referred to above is one which may be prepared by using, for example, 25 pounds of sulfuric :acid per cubic foot at a regeneration flow rate of between 2 and 25 gallons per minute per square foot of resin area. While the completely regenerated Iresin yields a sol having the most desirable finished pH, the regeneration necessary to produce such a resin is not always commercially expedient.

An incompletely regenerated strong cation exchange resin is one produced using more conventional regenerative procedures, e.g., 10 pounds of sulfuric acid per cubic foot at the flow rates corresponding to those used in producing a completely regenerated resin.

An incompletely regenerated cation resin, when used in combination with a hydroxide form strong base anion exchange resin in a mixed bed unit is capable of producing a low pH sol, e.g., pH 3 which upon storage will increase in pH.

When -a hydrogen form strong acid cation exchange resin is .used in combination with a free base form weak base anion exchange resin, the tinished sol treated wit-h this system will have a pH ranging between 3 and 4. The use of the weak base resin represents the least preferred ion exchange resin system, inasmuch as it does not remove acids completely.

When using both strong `acid cation and strong base anion exchange resins, care should be taken to have sufcient regeneration of the cation exchanger in order to prevent too high a pH in the final effluent. At pH values above 6, and especially above 7, the viscosity increase which develops in the strong base resin column will necessitate the use of high pumping pressures.

Of t-he three ion exchange resin systems discussed above, a hydrogen form strong acid cation exchange resin in combination with a hydroxide form strong base anion exchange resin is most desirable. This combination system is beneficial in removing weakly acidic materials such as carbonic and silicic acids, thereby adding to the stability and purity of the finished product.

Regardless of the starting ion exchange resin system used, the pH of the treated sols tends to increase. This increase in pH may be hypothesized as resulting from alkali metal ions which were initially occluded within the silica particles. Under acid conditions these alkali metal ions are removed from their associated state with the silica particles, and translocate into the aqueous phase of the sols. This migration of the alkali metal into the 1aqueous phase of the sol occurs during the aging step. The translocation of the alkali metal into the aqueous phase of the sols is further promoted by maintaining the sols under buffered acidic pH conditions. For this reason the pH of the sols during the aging process are desirably maintained at a pH between 1.5 Iand 3.5, and preferably between 2.5 and 3 by adding lto the sols after treatment with the .starting ion exchange resin system a water soluble low molecular weight acidic material, such as hydrochloric acid, acetic acid, sulfuric acid, phosphoric acid, and the like.

When using both cation and anion resins, addition of minor amounts of acid between the deionization steps can he accomplished by any one of several methods. In one such method the sol, after passage through the cation resin, is passed through an anion resin bed which leaks acid such as Ia weak base resin. By this means it is possible to incorporate the acid in the sol Without requiring a separate step. Another method is to bypass a portion of the cation bed effluent into the anion resin eiiiuent. In the more usual process, the acid is added to the aging tank after all the sol has passed through both columns.

The aging step, either alone or in conjunction with acidification of the ion exchange resin system treated sols, in promoting the migration of cations into the aqueous phase of t-he sols, allows the final deionization step of the invention to remove completely all vestiges of ionic components present thereby producing an ion free acidic sol of high purity and stability. When the aging process is conducted as indicated there is no change in the physical form of the silica particles nor is there any increase in the viscosity of the `linal sol. The -nished product is an aqueous liquid having colloidally dispersed therein substantially discrete, dense, nonagglomerated particles of silica. These pure salt-free sols have a pH ranging between 2.6-3.8 and preferably between 2.9 and y3.5, which pH is derived from the acidic surface sites of the colloidal silica particles. These stable sols show little or no change in pH under conditions of long-term storage, as evidenced by elevated `storage tests conducted at 120 F. for one month and for 20 weeks.

The nished sols may be further characterized `as having a specific conductivity less than 1,000 micromhos/ cm. at 77 yF. which is derived primarily from the dissociation of the silanol groups on the suraces of the silica particles.

The sols produced by thep ractices of the invention have t-he same silica content, eg., 3-50% as the starting 6 sols and have specific surface areas ranging between 50- 800 m.2/ g.

T he following examples will serve to illustrate the invention. The conductance measurements, abbreviated to mmhos, is the specic conductance in micromhos/cm. at 77 F.

EXAMPLE I A two-drum gallons) quantity of a commercially available 35% silica sol was deionized by passing the sol through a six inch diameter column containing 25 inches of a strong base anion exchange resin at the bottom of the column and 25 inches of a strong acid cation exchange resin at the top of the column. The strong base anion exchange resin was a commercially available product known as Nalcite SBR which is described in U.S. Patent 2,591,573. The strong acid cation exchange resin was the commercially available product Nalcite HCR which is described in U.'S. Patent 2,366,007. The deionization of the sol was carried out at a tlow rate of 0.75 -g.p.-m. (gallons per minute) which is equivalent to 5 g.p.m./ft.2. A composite of the efuent from the column produced a product having :a pH of 2.1 and a conductance of 650 mmhos. The low pH indicated that some acid leakage had occurred through the anion bed. After standing overnight, the material again was passed through the ion exchange unit. The composite of this material gave a pH of 3.10 and a conductance of 395 mmhos. This product is believed to be almost permanently stable.

EXAMPLE II In this experiment, a 70 gallon quantity of a commercially available 35% silica sol was deionized by passing the material through a six inch diameter column containing 0.324 ft.3 Nalcite SBR-OH- on the bottom and 0.337 ft.3 Nalcite HCR-H+ on the top at a iiow rate of 0.5 g.p.m. which is 3 g.p.m./ft.2. At frequent intervals during the preparation of this batch the pH, conductance, and soluble silica content of the product were measured. These measurements are set forth graphically in FIG. 1 of the drawing.

The average pH and conductance of the sol produced in this experiment were 2.9 and 500 mmhos, respectively. After standing overnight the pH of the sol rose to 3.5 and the conductance dropped to mmhos. After 11 days the pH increased to 5.15. After 23 days the pH of the sol was 5.32 and the viscosity was 11.96 cps. After one month the viscosity had reached 20.8 cps. and shortly thereafter the material gelled.

EXAMPLE III In this test, 43 gallons of a commercially available 35% silica sol was passed through a column containing the resins described in connection with Examples I and II. The ow rate of the sol was varied from 1.5 gpm/ft.'A7 to 6.2 g.p.m./ft.2. The pH and conductance of the product were not influenced by the variation in flow rate and were constant at 2.8 and 300, respectively. After one day, the pH of the sol rose to 3.30 and the conductance dropped to 110 mmhos. Portions of this sol (one day old and at pH 3.3) were treated in the following ways:

(A) Untreated.

(B) Deionized again.

(C) Sample adjusted to pH 1.0 with 16,000 p.p.m. HCl

and deionized after 4 hours.

(D) Sample adjusted to pH 2.0 with 1,600 p.p.m. HCl

and deionized after 4 hours.

(E) Sample adjusted to pH 2.0 with 1,600 p.p.m. HCl

and deionized after 3 days.

(F) Sample adjusted to pH 3.0 with 200 p.p.m. H3PO4.

(G) Sample adjusted to pH 3.0 with 50 p.p.m. H2804.

FIG. 2 of the drawing shows the effect of the above treatments on the pH of the sol. As can be seen from the drawing, double deionization even without the addition of acid stabilizes the product at the desired pH. The addi- 7 tion of acid (50 ppm. H2504 and 200 p.p.m. H3PO4) is ineifective in preventing an increase in the viscosity of the sol.

One of the accepted theories of colloidal chemistry is t the step of placing the sol in ion exchange relationship with an anion exchange resin, the steps of: placing said sol in ion exchange relationship with a strong acid cation exchange resin in the hydrogen form; removing said sol that the gelation of silica sols is caused by the condensa- 5 from contact with said cation exchange resin; aging said tion of silanol groups on the surface of the particles which sol out of contact lwith said cation exchange resin for a forms siloxane linkages. The fewer collisions between the period of at least about one hour at a temperature varyindividual particles, the less likely is it that the silanol ing from about 34 F. to about the boiling point of said groups of different particles will react to form siloxane sol; and again placing the said sol in ion exchange relabonds. The particles tend to remain apart because of the lo tionship with a strong acid cation exchange resin in the negative charge on their surface. It is known that OH hydrogen form. ions in the solution catalyze the -condensation of silanol 2. An improvement in a process for stabilizing a silica groups. It is also known that neutral salts which are pressol having an SiO2 content of from about 3% to about ent in the solution reduce the repulsive force exerted by 50% wherein said sol is placed in ion exchange relationthe charged particles. One of the distinct advantages of ship with an anion exchange resin and a -cation exchange the present invention is that substantial amounts of neuresin, the improvement which comprises in addition to tral salts are not formed during the stabilization step. the step of placing the sol in ion exchange relationship As was pointed out above, it is likely that the increase with an anion exchange resin, the steps of: placing said in pH of the deionized sol is due to the presence of ocsol in ion exchange relationship with a strong acid cation cluded sodium ions. Viscosity studies have shown that exchange resin in the hydrogen form; removing said sol there is an occluded volume of about 5% in the sol parfrom contact with said cation exchange resin; aging said ticle. By allowing the product to stand for at least four sol out of contact with said cation exchange resin for a hours and preferably for at least 14 hours the bulk of the period of at least about one hour at temperatures varysodium ions diffuse out into the liquid where the second ing from about 120 F. to about the boiling point of said deionization will remove them. It has been found that this sol; and again placing the said sol in ion exchange reladouble ion exchange treatment produces a product contionship with a strong acid cation exchange resin in the taining insufficient residual sodium to increase the pH of hydrogen form. the sol above about 3 2. 3. An improvement in a process for stabilizing a silica It would, of course, be possible to pass the sol more sol having an SiO2 content of from about 3% to about than twice through the deionization bed. Such a proce- 3() 50% wherein said sol is placed in ion exchange relationdure, however, would be impractical from a commercial ship with an anion exchange resin and a cation exchange standpoint and would not increase the stability of the sol resin, the improvement which comprises in addition to to any substantial extent. the step of placing the sol in ion exchange relationship Under certain conditions, it has been found to be dewith an anion exchange resin, the steps of: placing said sirable to add minor amounts of an acidic material after sol in `ion exchange relationship with a strong acid cation the second deionization step. These acidic materials may exchange resin in the hydrogen form; removing said sol be selected from low molecular weight acids such as hyfrom Contact With Sad CatOn eXChnnge resin; aging Said drochloric, sulfuric, acetic and phosphoric with the latsol out of contact with said cation exchange resin for ter two being the preferred members of this class. The a period of at least about 4 hours at a temperature of amount added vusually will not be more than 50 parts per 40 from about 34 F. to about the boiling point of said sol; million, and preferably 10-30 parts per million. These and again placing the said sol in ion exchange relationacids tend to neutralize any metallic corrosion products ship with a strong acid cation exchange resin in the hyor traces of alkalinity leached from drum or tank coatdrogen form. ings during the storage and transportation of the sols. 4. An improvement in a process for stabilizing a silica The following table sets forth sols which exemplify sol having an SiO2 content of from about 3% to about the materials which can be treated by the subject process. wherein said sol is placed in ion exchange relation- NALooAG NALCOAG NALCOAG NALCOAG 1015 1030 1035 1050 Percent colloidal Silica as SiOz 15 30 35-35 49-50 ILHN.. 8. 6 10. 2 8. 6 9. 0 Viscosity at 77 F. cps 5 5 5 30 Specific gravity at 68 F 1.09 1. 205 1. 255 1. 385 Average surface area, M2/gram of SiO B30-430 190-270 135-190 1Z0-150 Average particle size, mil1imicrons 7-9 11-16 16-22 20-25 Density, #/gauon at 68 F 9.1 10.0 10.5 11.6 Freezing point, deg. F. 32 32 32 32 N azO percent 0. O4 0. 40 0. 10 0` 3 The holding period between the deionization steps can ship with an anion exchange resin and a cation exchange range from 1 hour up to as long as one week or more. resin, the improvement which comprises in addition to Our preferred holding period is from 14 to about 48 the step of placing the sol in ion exchange relationship hours, and our most preferred holding period is from with an anion exchange resin, the steps of: placing said 16 to 24 hours. sol in ion exchange relationship with a strong acid cation Obviously many modiiications and variations of the exchange resin in the hydrogen form; removing said sol invention as hereinbefore set forth may be made without from contact with said cation exchange resin; aging said departing from the spirit and scope thereof, and therefore sol out of contact iwith said -cation exchange resin for a only such limitations should be imposed as are indicated period of at least about 4 hours at a temperature of from in the appended claims. about 40 F. to about 120 F.; and again placing the We claim: said sol in ion exchange relationship with a strong acid 1. An improvement in a process for stabilizing a silica cation exchange resin in the hydrogen for-m. sol having an SiO2 content of from about 3% to about 5. An improvement in a process for stabilizing a silica 50% wherein said sol is placed in ion exchange relationsol having an SiO2 content of from about 3% to about ship with an anion exchange resin and a cation exchange 50% wherein said sol is placed in ion exchange relationresin, the improvement which comprises in addition t0 75 ship with an anion exchange resin and a cation exchange resin, the improvement which comprises in addition to the step of placing the sol in ion exchange relationship with an anion exchange resin, the steps of: placing said sol in ion exchange relationship with a strong acid cation exchange resin in the hydrogen form; removing said sol from contact with said cation exchange resin; aging said sol out of contact with said cation exchange resin for a period 4ranging from 14 to 48 hours at a temperature of from about 34 F. to about the boiling point of said sol; and again placing the said sol in ion exchange relationship with a strong acid cation exchange resin in the hydrogen form.

6. An improvement in a process for stabilizing a silica sol having an SiO2 content of from about 3% to about 50% wherein said sol is placed in ion exchange relationship with an anion exchange resin and a cation exchange resin, the improvement which comprises in addition to the step of placing the sol in ion exchange relationship with an anion exchange resin, the steps of: placing said sol in ion exchange relationship with a strong acid cation exchange resin in the hydrogen form; removing said sol from contact with said cation exchange resin; aging said sol out of contact with said cation exchange for a period ranging from 16 to 24 lhours at a temperature of -from about 40 F. to about 120 F.; and again placing the said sol in ion exchange relationship with a strong acid cation exchange resin in the hydrogen form.

7. An improvement in a process for stabilizing a silica sol having an SiO2 content of from about 3% to about 50% wherein said sol is placed in ion exchange relationship with an anion exchange resin and a cation exchange resin, the improvement which comprises in addition to the step of placing the sol in ion exchange relationship with an anion exchange resin, the steps of: placing said sol in ion exchange relationship with a strong acid cation exchange resin in the hydrogen form; removing said sol from contact with said cation exchange resin; buffering the pH of the sol with a water-soluble acidic material to produce a pH within the range of 1.5-3.5; aging said sol out of contact with said cation exchange resin for at -least about one hour at a temperature `between about 34 F. and the boiling point of said sol; and again placing the said sol in ion exchange relationship with a strong acid cation exchange resin in the hydrogen form.

8. An improved process as in claim 7 wherein said sol is aged at a temperature ranging between F. and F. for at least about 4 hours.

9. An improved process as in claim 7 wherein said sol is aged at a temperature between about 120 F. and F. for a period of time of from about one hour to about 3.5 hours and wherein said sol is buered within a pH range of 2.5 to 3.

10. A-n improvement in a process for stabilizing a silica sol having an Si02 content of from about 3% to about 50% which comprises: passing said sol through a column containing a strong acid cation exchange resin in the hydrogen form; aging said sol out of Icontact with said cation exchange resin for between about 14 to 48 hours at a temperature between about 40 F. and 120 F.; and thereafter passin-g said sol again through a column containing a strong acid cation exchange resin in the hy- `drogen form and through a column containing a strong :base anion exchange resin.

11. An improved process as in claim 10 wherein said sol is passed through a column containing a mixed bed of a strong acid cation exchange resin in the hydrogen form and a strong lbase anion exchange resin after the aging step in the process.

References Cited FOREIGN PATENTS 521,741 2/1956 Canada.

LEON D. ROSDOL, Primary Examiner.

.TULIUS GREENWALD, Examiner.

R. D. LOVERING, Assistant Examiner. 

1. AN IMPROVEMENT IN A PROCESS FOR STABILIZING A SILICA SOL HAVING AN SIO2 CONTENT OF FROM ABOUT 3% TO ABOUT 50% WHEREIN SAID SOL IS PLACED IN ION EXCHANGE RELATIONSHIP WITH AN ANION EXCHANGE RESIN AND A CATION EXCHANGE RESIN, THE IMPROVEMENT WHICH COMPRISES IN ADDITION TO THE STEP OF PLACING THE SOL IN ION EXCHANGE RELATIONSHIP WITH AN ANION EXCHANGE RESIN, THE STEPS OF PLACING SAID SOL IN ION EXCHANGE RELATIONSHIP WITH A STRONG ACID CATION EXCHANGE RESIN IN THE HYDROGEN FORM; REMOVING SAID SOL FROM CONTACT WITH SAID CATION EXCHANGE RESIN, AGING SAID SOL OUT OF CONTACT WITH SAID CATION EXCHANGE RESIN FOR A PERIOD OF AT LEAST ABOUT ONE HOUR AT A TEMPERATURE VARYING FROM ABOUT 34*F. TO ABOUT THE BOILING POINT OF SAID 